Which of the following is the most common site of origin for atrial septal defects (ASDs)?

Explanation

## Atrial Septal Defect Classification and Epidemiology **Key Point:** Ostium secundum ASD accounts for approximately 70–75% of all ASDs, making it the most common type. ### Embryological Basis Ostium secundum defects result from inadequate growth of the **septum secundum** and/or excessive resorption of the **septum primum**. These defects occur in the region of the fossa ovalis, which is the normal site of the foramen ovale. ### Comparison of ASD Types | ASD Type | Frequency | Embryological Defect | Location | Clinical Significance | |----------|-----------|----------------------|----------|----------------------| | **Ostium Secundum** | 70–75% | Inadequate septum secundum growth; excessive septum primum resorption | Fossa ovalis region | Most common; usually well-tolerated; may present late | | **Ostium Primum** | 15–20% | Failure of endocardial cushion fusion | Lower atrial septum; extends to AV valve region | Associated with cleft mitral valve; earlier symptoms | | **Sinus Venosus** | 10–15% | Abnormal development of sinus venosus; inadequate septation | Near entry of SVC or IVC | Often associated with partial anomalous pulmonary venous return | | **Unroofed Coronary Sinus** | <1% | Absent wall between coronary sinus and left atrium | Coronary sinus region | Rare; may cause left-to-right shunt | **High-Yield:** Ostium secundum is the **default answer** when the question asks for "most common ASD" without further specification. ### Clinical Pearl Ostium secundum ASDs are often discovered incidentally in adulthood because they may be well-tolerated for decades. The left-to-right shunt is typically modest, and symptoms (if any) develop gradually with age as pulmonary vascular resistance increases. ### Mnemonic **"POSS"** — **P**rimary (ostium primum), **O**stium secundum (most common), **S**inus venosus, **S**inus (coronary sinus unroofed). - Ostium secundum = **70–75%** (remember: "**7** out of **10** ASDs") [cite:Robbins 10e Ch 12]